In rechargeable electrochemical cells weight and portability are important considerations. It is also advantageous for those cells to exhibit long operating life and be capable of operating without periodic maintenance. They may be used as direct replacements for primary AA, C and D cells in numerous consumer devices such as calculators, portable radios and flashlights. They are more often integrated into a sealed power pack designed to interface with or be an integral part of a specific device.
The rechargeable electrochemical cell is ideally suited to serve as a portable power source due to its small size, lightweight, high power capacity and long operating life. The rechargeable cell must operate as an "install and forget" power source. With the exception of periodic charging, a rechargeable cell should perform without attention, rarely becoming the limiting factor in the life of the device it powers.
Secondary cells using a rechargeable hydrogen storage negative electrode are known in the art. See, for example, U.S. Pat. No. 4,551,400 for HYDROGEN STORAGE MATERIALS AND METHODS OF SIZING AND PREPARING THE SAME FOR ELECTROCHEMICAL APPLICATIONS, the disclosure of which is incorporated herein by reference. Hydrogen storage cells operate in a different manner from lead-acid, nickel-cadmium or other prior art battery systems. Hydrogen storage electrochemical cells utilize a metal hydride negative electrode that is capable of reversibly electrochemically storing hydrogen. In one embodiment, the cell employs a positive electrode formed of nickel hydroxide material, although other positive electrode materials may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte, and may include a suitable separator, spacer, or membrane therebetween.
Upon application of electrical current to the negative electrode, the negative electrode material (M) is charged by the electrolyte decomposition of water to form the hydride and a hydroxide (OH) ion: EQU M+H.sub.2 O+e.sup.- M--H+OH.sup.- (Charging)
Upon discharge, the hydride is decomposed, to release hydrogen within the cell, reacting with OH-ion to form water, and releasing an electron to the external circuit to provide an electric current: EQU M--H+OH.sup.- M+H.sub.2 O+e.sup.- (Discharging)
The negative electrode reactions are reversible.
The reactions that take place at the positive electrode are also reversible. For example, the reactions at a conventional nickel hydroxide positive electrode as utilized in a hydrogen rechargeable secondary cell or battery are: ##STR1##
Hydrogen storage negative electrode cells are designed to operate with a nickel hydroxide positive electrode and a hydrogen storage alloy negative electrode, separated by non-woven, felted, nylon or polypropylene separator. The electrolyte is generally an alkaline electrolyte, for example, 20 to 45 weight percent potassium hydroxide, where lithium hydroxide may also be present in limited quantity.
A cell utilizing an electrochemically rechargeable hydrogen storage negative electrode offers important advantages over conventional secondary cells. Rechargeable hydrogen storage negative electrodes offer significantly higher specific charge capacities (both in terms of ampere hours per unit mass and ampere hours per unit volume) than do either lead negative electrodes or cadmium negative electrodes. As a result of the higher specific charge capacities, a higher energy density (in terms of watt hours per unit mass or watt hours per unit volume) is possible with hydrogen storage battery than is possible with the prior art systems, thereby making hydrogen storage cells particularly suitable for many commercial applications. Hydrogen storage cells are of two types, sealed cells and vented cells. In addition to differences in basic construction, as implied by the nomenclature, the two types of cells differ in their modes of operation. During normal operation, a sealed cell does not permit the venting of gas to the atmosphere. By way of contrast, in a vented cell, venting may be part of the normal operating behavior. As a result of this difference, the vent assemblies associated with each type of cell are quite different from one another, and the amounts of electrolyte in the cell container relative to the electrode geometry differ significantly.
Sealed cells are generally manufactured in many configurations, predominantly including cylindrical and rectangular. Sealed cells are designed to operate in a starved electrolyte configuration. That is, sealed cells are designed to operate with a minimum amount of electrolyte. The cell enclosure for a sealed cell is normally a metal enclosure designed for operation of up to about 100 pounds per square inch absolute or even higher. Sealed cells are characterized by the substantial absence of any required maintenance, though "one time" venting cells require some periodic maintenance.
By way of comparison, vented cells, which have a nickel hydroxide positive electrode, and a hydrogen storage alloy negative electrode, typically have a woven or nonwoven separator, e.g., of nylon or polypropylene. The vented cell differs most significantly from the sealed cell in that it is operated in a flooded condition. A "flooded condition" is defined herein to mean that the electrodes are completely immersed in, covered by, and wetted by the electrolyte. Thus, such cells are sometimes referred to as "flooded cells." A vented cell can be further distinguished from a sealed cell in that the vented cell is designed for normal operating pressures of only up to about 25 pounds per square inch, after which excess pressures are relieved by a vent mechanism.
The discharge capacity of the nickel positive electrode is limited by the amount of electrolyte, the amount of active material and charging efficiencies. The charge capacity of the negative, hydrogen storage alloy electrode is limited by the amount of active material used since its charge efficiency is nearly 100 percent until a nearly full state of charge is reached. To maintain the optimum capacity of the hydrogen storage electrode, precautions must be taken to avoid oxygen recombination or hydrogen evolution before full charge is reached. This is generally accomplished by providing an excess of negative electrode material. However, precautions must be taken in the design and fabrication of sealed cells to avoid effects of over-pressurization associated with overcharge at dangerously high charge rates.
Typically, rechargeable sealed cylindrical cells use a cylindrical nickel-plated steel case as the negative terminal and the cell cover as the positive terminal. The electrodes, are wound to form a compact "jelly roll" with the electrodes of opposite polarity isolated from each other by a porous separator. An insulator separates the positive cover from the negative cell can.
Conventional, cylindrical, rechargeable cell construction relies upon a tab which extends from one point on each plate. This creates a single current path through which current must be distributed to the entire electrode area during charging and discharging.
While rechargeable cells can be used without deleterious effect in most applications, there are many instances in which damage, of the type described hereinabove, may occur to the cells. For instance, during periods of abusive overcharging for extended lengths of time, excess pressure can be generated interiorly of the cell can. As the internal pressure increases, so also does the danger of an explosive failure. Accordingly, some means must be provided to safely release excess pressure, thereby avoiding the unsafe condition of container failure, which may be referred to as rupture.
Cylindrical, rechargeable cells of the prior art included a "one time only" venting mechanism where, for example, a rupturable diaphragm and blade apparatus was employed. As internal cell pressure increased, the blade was forced against the diaphragm. As the pressure further increased, the blade punctured the diaphragm, allowing excess gases to escape through the ruptured diaphragm. This destructive type of venting mechanism was both unpredictable from batch to batch and from cell to cell within a batch. Moreover, destructive venting is good for only one excess pressure situation. After the diaphragm is punctured it cannot even sustain normal cell operating pressures. In one embodiment of such "one time only" venting, the diaphragm is punctured by an upward driven plunger. In another embodiment, the diaphragm is forced upward towards the incising blade. As the internal cell pressure reaches the critical level, i.e., as the diaphragm reaches a critical deformation, the diaphragm is forced against the incising blade which would thus puncture the diaphragm, allowing the escape of accumulated cell pressure. An example of this type of device is fully disclosed in U.S. Pat. No. 3,415,690, the disclosure of which is incorporated herein by reference.
One shortcoming present in this and other methods of venting rechargeable electrochemical cells is that the venting mechanism present therein was a "one time only" mechanism. Once vent integrity is broken and the electrolyte material inside said rechargeable cell is exposed to the surrounding atmosphere, the electrolyte levels would be disturbed and thus the ability of the cell to retain and dispense an electrochemical charge would be deleteriously effected. Another shortcoming was the dependence of venting on the movement of a thin, deformable diaphragm against the incising blade.
Other prior art venting assemblies included square or polygonal shaped rubber stoppers, attached to a vented plate at three of the square's four corners. This assembly proved beneficial in that it was not a "one time only" mechanism. Unfortunately, this polygonal assembly either failed to reliably vent at a given internal cell pressure or "stuck" open, thereby causing the evaporation of the electrolyte material. This failure is attributable to the formation of a "pocket" by the stopper which prevented cell gases from properly venting. The result, of course, was failure of the rechargeable cell. An example of this type of ventable cover assembly is fully disclosed in U.S. Pat. No. 3,944,749, to Decker, et al, the disclosure of which is incorporated herein by reference.
Unfortunately, cell cover assemblies such as those disclosed by Decker, et al, or in U.S. Pat. No. 4,298,662 to Sugalski, et al are adapted for use in nickel-cadmium (Ni-Cd) cells, which cells do not evolve hydrogen at either the positive or negative electrode. Accordingly, the vent septum employed to seal the vent orifice of Ni-Cd cells is not capable of preventing the escape of hydrogen gas from the interior of metal-hydride hydrogen storage alloy electrochemical cells. Indeed, the present inventors have found that the ethylene propylene diene monomer (EPDM) commonly employed for the fabrication of vent septums in Ni-Cd cells (see for example Sugalsi, et al, Column 5, lines 53-57) is quite hydrogen permeable at pressures well below the designed vent release pressure, and is therefore wholly inadequate for purposes of preventing hydrogen leakage from hydrogen storage electrochemical cells. It is important to note that, as used herein, the term "hydrogen impermeable" refers not only to the loss of hydrogen occasioned by the diffusion of hydrogen through the vent system, but also the loss of hydrogen resulting from inadequate sealing between the vent septum and the vent orifice, thus allowing hydrogen to seep therethrough.
In selecting an appropriate hydrogen impermeable material, several factors must be taken into account, for example:
1. Thin profile for minimum compression deformation; PA1 2. Elasticity for maximum conformation to a sealing surface; PA1 3. Low coefficient of hydrogen permeation; PA1 4. Resiliency for surviving the geometric deformation induced by enclosure and rupture from pressure; PA1 5. Atmospheric compatibility to elevated caustic vapors; and PA1 6. Temperature compatibility to withstand the elevated temperature experienced in many situations.
The only material suitable for accomplishing factors 1, 2, and 4-6 is silicone. However, silicone is woefully inadequate for purposes of preventing hydrogen permeation.
Accordingly, there exists a need for a sealable cover assembly including a venting mechanism whereby excessive internal cell pressures can be vented off or released from the rechargeable metal-hydroxide hydrogen storage alloy cells at predictable pressures without destruction of the cells, as by permanently exposing the electrolyte inside the electrochemical cells to ambient conditions. Especially important is the requirement that the venting mechanism include a vent septum which is hydrogen impermeable, i.e., forms a seal with the vent orifice which resists hydrogen leakage.